Object-Oriented Programming

Duration: 7.0 min

Object-Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm based on objects and classes. It's the foundation of Java and enables you to write modular, reusable, and maintainable code. OOP principles include encapsulation, inheritance, polymorphism, and abstraction. Understanding OOP is crucial for professional Java development and building large-scale applications.

public class Car {
    private String brand;
    public Car(String brand) { this.brand = brand; }
    public void display() { System.out.println("Brand: " + brand); }
}
public class Main {
    public static void main(String[] args) {
        Car car = new Car("Toyota");
        car.display();
    }
}

Brand: Toyota

class Animal {
    void eat() { System.out.println("Eating"); }
}
class Dog extends Animal {
    void bark() { System.out.println("Barking"); }
}
public class Test {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.eat();
        dog.bark();
    }
}

Eating
Barking

class Shape {
    void draw() { System.out.println("Shape"); }
}
class Circle extends Shape {
    @Override void draw() { System.out.println("Circle"); }
}
class Rectangle extends Shape {
    @Override void draw() { System.out.println("Rectangle"); }
}

Polymorphism example

public class Person {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}

Encapsulation example

public class Student {
    private String name;
    public Student() { this.name = "Unknown"; }
    public Student(String name) { this.name = name; }
}

Constructor example

public class Counter {
    static int count = 0;
    Counter() { count++; }
    static void display() { System.out.println("Count: " + count); }
}
public class Main {
    public static void main(String[] args) {
        new Counter();
        new Counter();
        Counter.display();
    }
}

Count: 2

interface Animal {
    void sound();
}
class Dog implements Animal {
    public void sound() { System.out.println("Woof"); }
}
class Cat implements Animal {
    public void sound() { System.out.println("Meow"}; }
}

Interface example

abstract class Vehicle {
    abstract void start();
    void stop() { System.out.println("Stopped"); }
}
class Car extends Vehicle {
    void start() { System.out.println("Car started"); }
}

Abstract class example

public class Example {
    int x = 10;
    void display() {
        int x = 20;
        System.out.println("Local: " + x);
        System.out.println("Instance: " + this.x);
    }
}

Local: 20
Instance: 10

class Parent {
    void display() { System.out.println("Parent"); }
}
class Child extends Parent {
    void display() {
        super.display();
        System.out.println("Child");
    }
}

Parent
Child

public class FinalExample {
    final int MAX = 100;
    final void display() { System.out.println("Final method"); }
}
final class ImmutableClass { }

Final keyword example

💡 Tip: Best Practice: Use encapsulation to protect your data. Make fields private and provide public getter/setter methods.

💡 Tip: Common Mistake: Forgetting to use @Override annotation when overriding methods. This helps catch errors at compile time.

Learn more: https://docs.oracle.com/javase/tutorial/java/concepts/